1. Field of the Invention
The present invention relates to a nozzle box that constitutes a channel of a working fluid leading the working fluid to a first-stage nozzle of an axial flow turbine, and to an axial flow turbine including the nozzle box.
2. Description of the Related Art
An axial flow rotary machine such as a steam turbine used in a thermal power station and the like includes blade cascades composed of a plurality of stages of the combination of a nozzle whose channel for the passage of a working fluid is stationary and a rotor blade which rotates. A steam turbine is generally divided into a high-pressure part, an intermediate-pressure part, and a low-pressure part depending on a condition of steam being a working fluid. In order to improve efficiency of the work by the working fluid in each blade cascade part, channels between the blade cascades have to be designed in a shape allowing smooth flow of the working fluid.
Conventionally, in power generating machines, efficiency improvement of the machines has been an important task in order to realize effective use of energy resources and reduction in CO emission. An example of a measure to improve efficiency of a steam turbine is to effectively convert given energy to mechanical work. One measure for this is to reduce various internal losses.
The internal losses in a steam turbine blade cascade of a steam turbine include a profile loss ascribable to the shape of blades, a secondary loss ascribable to a secondary flow, a leakage loss ascribable to leakage of a working fluid to the outside of a blade cascade, and a moisture loss ascribable to drain, which is unique to a final blade group. The internal losses further include a loss in a steam valve, a passage part leading steam to some blade cascade, and a passage part from some blade cascade up to the next blade cascade, an exhaust loss in a low-pressure final stage, and so on.
For example, JP-A 2008-38741 (KOKAI) discloses an art to uniformly lead a working fluid to a blade cascade in order to reduce a pressure loss in a passage part connecting some blade cascade and another blade cascade. According to this art, in order to uniformly lead the working fluid to a blade cascade of an axial flow turbine, the width of the passage part through which the working fluid passes is monotonously increased toward a downstream side.
Here, the structure of a conventional nozzle box 300, which is a working fluid (e.g. steam) inlet of an axial flow turbine, will be described. FIG. 9 is a perspective view showing part of the conventional nozzle box 300. FIG. 10 is a view showing the conventional nozzle box 300 in its cross section vertical to a turbine rotor seen from a first-stage nozzle 303 side. FIG. 11 is a view showing a cross section of the conventional nozzle box 300 taken along a channel center line. The illustration of the turbine rotor, which is penetratingly provided at the center of the nozzle box 300, is omitted here.
For example, as shown in FIG. 9, the nozzle box 300 is a structure forming a steam channel through which steam led into lead-in pipes 302 passes to be led into a first-stage nozzle 303.
As shown in FIG. 10, the nozzle box 300 is separated into two upper and lower spaces, and steam 301 from a boiler (not shown) is led into each of the spaces through the two lead-in pipes 302.
As shown in FIG. 10, the steam 301 led into the lead-in pipes 302 made of a cylindrical pipe is led to the first-stage nozzle 303 through an annular channel 304. On a downstream side of the first-stage nozzle 303, the whole periphery of the passage part is coupled, and the steam 301 having passed through the first-stage nozzle 303 is led to a first-stage rotor blade (not shown).
Here, Sa-1 to Sn-1 shown in FIG. 10 each are a steam channel width in a first direction intersecting with a channel center line 305 at a predetermined position of a steam channel formed by the nozzle box 300. Sa-2 to Sn-2 shown in FIG. 11 each are a steam channel width in a second direction intersecting with the channel center line 305 and perpendicular to the first direction. The steam channel width in the first direction and the steam channel width in the second direction exist on the same channel cross section perpendicularly intersecting with the channel center line 305 of the steam channel. Further, when the seam channel width in the first direction and the steam channel width in the second direction are different from each other, the steam channel width in the first direction is a steam channel width in a longitudinal direction on the channel cross section. That is, the steam channel width in the first direction is the largest channel width on this channel cross section.
As shown in FIG. 9, for example, at an inlet portion of the nozzle box 300, a cross sectional shape of the steam channel is circular. Therefore, the steam channel width in the first direction and the steam channel width in the second direction are equal to each other. Here, a steam channel width in a direction corresponding to a steam channel width in the longitudinal direction of a channel cross section which is on a downstream side of the cross section where the cross sectional shape of the steam channel is circular and thus the steam channel width in the first direction and the steam channel width in the second direction are different from each other, is set as Sa-1. Further, the steam channel width in the first direction intersecting with the channel center line 305 at an outlet of the nozzle box 300, that is, at an inlet of the first-stage nozzle 303 is shown as Sn-1, and the steam channel width in the second direction intersecting with the channel center line 305 and perpendicular to this first direction is shown as Sn-2.
In the conventional nozzle box 300, as shown in FIG. 10, the steam channel width Sa-1 and the steam channel width Sb-1 in each of the lead-in pipes 302 are equal to each other, but the steam channel width begins to widen from the steam channel width Sc-1 near a joint portion between the lead-in pipe 302 and the annular channel 304. The steam channel widths Sd-1, Se-1 in the annular channel 304 greatly widen further. Further, as shown in FIG. 11, the steam channel width Sa-2 to the steam channel width Sc-2 in the lead-in pipe 302 are equal to one another, but the steam channel width gets gradually narrower from the steam channel width Sd-2. Then, the steam channel width Sn-2 at the inlet of the first-stage nozzle 303 is equal to the height of the first-stage nozzle 303.
FIG. 12 is a graph showing area ratios equal to areas of channel cross sections Sa to Sn which include the steam channel widths Sa-1 to Sn-1, Sa-2 to Sn-2 shown in FIG. 10 and FIG. 11 and perpendicularly intersect with the channel center line 305 of the steam channel, divided by an area of the channel cross section Sa which is at the inlet of the lead-in pipe and which includes the steam channel widths Sa-1 and the steam channel width Sa-2 and perpendicularly intersects with the channel center line 305 of the steam channel. Note that FIG. 12 also shows area ratios in channel cross sections other than the channel cross sections Sa to Sn.
As shown in FIG. 12, the area ratios of the channel cross sections up to a channel cross section slightly on an upstream side of the channel cross section Sc have a constant value of 1 since they are channel cross sections of the aforesaid lead-in pipe 302. In the channel cross sections on a downstream side of the channel cross section slightly on the upstream side of the channel cross section Sc, the area ratio abruptly increases. The area ratio presents a peak in the channel cross section Sd, and the area ratio abruptly decreases in the channel cross section on a downstream side of the channel cross section Sd.
FIG. 13 is a graph showing a total pressure loss ratio in each of the channel cross sections shown in FIG. 12. Here, the total pressure loss ratio is expressed by the following expression (1), where Pa is a total pressure in the channel cross section Sa at the inlet of the steam channel formed by the nozzle box 300 and Po is a total pressure in a given channel cross section.total pressure loss ratio (%)=(Pa−Po)/Pa×100  Expression (1)
Note that the above total pressure loss ratios are obtained by three-dimensional thermal-fluid analysis in a steady state by using a CFD (Computational Fluid Dynamics).
As shown in FIG. 13, the total pressure loss ratio abruptly increases from the channel cross section slightly on the upstream side of the channel cross section Sc. This is a pressure loss that occurs because, from the channel cross section slightly on the upstream side of the channel cross section Sc, the steam channel width abruptly increases and thus the area ratio abruptly increases as shown in FIG. 12.
As described above, the conventional nozzle box 300 in the axial flow turbine has the problem that the abrupt increase in the area ratio due to the abrupt increase in the steam channel width causes a great pressure loss. This lowers turbine efficiency of the axial flow turbine and thus makes it difficult to obtain high turbine efficiency.